Demodulator and method op demodulation



JAN. 8, 1929. 1.698.668 S. BALLANTINE ET AL DEMODULATOKR AND METHOD 0FDEMQDULATION Filed June 8. 1923 @ffl 4 Sheets-Sheet 1 lef/'ned S27/kgaGHG: new.

Jams, 1929. 1,698,668 -S. BALLANTINE ET AL DEMODULATOR AND METHOD OFDEMODULATION Filed June 8.V 1523 4 Sheets-Sheet 2 Jan. s, 1929.1,698,668

S. BALLANTINE ET AL DEMODULATOR -AND METHOD OF DEMODULATION Filed June8. 1.923

. 4 Sheets-Sheet 5 Jan. 8, 1929. 1,698,668 i s. BLLANTINE ET AL YDEMODULATOR AND' METHOD oF DEMODULATION Filed June 8. 1923 4 Sheetssheet 4 33?? www( 1M Patented Jan. e; 1929.-

UNITED sr-Aras PATENT oFFIcE.

` STUART BALL'ANTINE AND LEWIS u. HULL,

or BooNToN, :mw mam, AssIGNoas '120l RADIO FREQUENCY LABORATORIES,ITNCORIORA'JED,v OF BOONTON, NEW .TER- SEY; A CORPORATION' OFNEW-JERSEY.

'- DEMonULAToa AND METHOD or nnMonULATIoN l Bussum Appucaon mea :une s',l192s. serial No. 344,215.

lThis invention relates to demodulators for use in radio receivingcircuits, particularly thoseused for radio telephony, and to methods ofdemodulation.

An object ofthe invention is-to provide a demodulator which is free fromdistortion.

A further'object of the invention is to `provide a method ofdemodulation by which the audio frequency output current mafy be madeproportional to the rst power o the radio frequency amplitude.- Afurther object is to provide a method of demodulation-by l which thestimulus-response -characteristic of the demodulator may be adjusted to've a true and undistorted copy of the modu ating currents. operating inthe' transmitter, and to adjust the tonal characteristics of the soundsproduced.

The usual system of demodulation is based circuit element or device,

upon the use of -a usually called a detector, which possesses anasymmetrically conducting property. The essential property` of thedetector, in prior art, is not a true unilateral conductivity, resultingin simple rectification, but the property of asymmetrical conductivitycorresponding to the curvature of its currentvoltage characteristic. Ithas been recognized that the audio-frequencyl current o tained in priordevices is not a true 'copy of the original modulation of the signal andit can be demonstrated that thedetected audiofrequency current is not ofa lower order than the second power of the impressed signal voltage.

According to the present invention, however, the old types of detectorswhich rely solely upon asymmetrical conductivity are abandoned and weemploy instead a detecting element having a certain operating range inwhich the response is directly-proportional to the amplitude of theimpressed voltage stimulus. Thus by means of our invention we are ableto obtain in response to arnodulated radio frequency signal voltage anaudio frequency current whose amplitude is directly `proportional to theamplitude of said radio frequency voltage.

' tained between the stimulus impressed upon our demodulating quencyresponse stands in contrast with the square-law relation`characteristics of ionic tube and crystal detectors as commonly employedand results in a marked improvement voltage over This linear relationobsubstance, device and the audio fre-.-

' ly used in the detection of signals.

Fig. 2 is a diagrammatic representation y of static, current-voltagecharacteristics of a detector element having substantially constantconductivity over a wide range.

Fig..- 3 is a diagrammatic representation of experimental responsecharacteristics for adetector element having current-voltagecharacterlstics as shown in Fig. 2.

Fig; 4 is a diagrammatic representation of the variation of the exponentin the law of response for a detector element having a responsecharacteristic such as shown 1n Fig. 3; and

Figs. 5,'and 6 are diagrams of apparatus embodying our invention.

-.The experimental data which is represented in the curves of Fig. 1 wasobtained by measurements taken on an asymmetrical conductor formed by alight metallic pointcontact upon a crystal of refined silicon. Thecharacteristic curves were obtained for three different points byimpressing a.A steady but reversible voltage u n the" detector andmeasuring the resulting current in the two directions for direct andreversed voltage. When operated 'by a radio-frequency si al a smallrange on one .of t ese curves the direct current and audio-frequencycurrent components are greater2 the greater the curvature ofthepcharacteristic. Accordingly the characteristic marked 3 representsan operatingcondition more favorable for detection than that marked 2; 2represents a more favorable condition than 1, where the detector ispractically a simple conductor, without appreciable de properties. Thecurvature and general crm. of the current-voltage characteristics dependfirst upon the com 'tion of the crystalline and second, upon thelocation and pressure of the contact' point. Locations upon thecrystalsurface at which the current-voltage curvecxlitbly asymmetricalare commonly charac as sensitlve spots in the crystal and canbe locatedby connecting the crystal series with a telephone recelver and with asource of modulated radiorequency voltage and exploring the crystalwhile listening for the sound which indicates a detection of themodulated wave. It was by this method that the spots whosecharacteristics are shown at 2 and 3 were located.

The audio-frequency current which is -.passed by an asymmetricalconductor such as described is afunction of the voltage and maybeexpressed as =f(re). The absolute strenvth o the signal. is small andthe device may be polarized by means of an auxiliary steady E. M. F. soadjusted that operation takes place about the point Eo on thecharacteristic curve 3 of Fig. 1. Then expanding z' in a power seriesabout this point, denoting the difference between E and E,l by e, wehave:

where the symbols f', f, indicate the first, second and thirdderivatives, respectively, of i with respect to e, taken at the point o.Now a radio telephone signal is impressed upon the detector and forsimplicity this signal will be taken as of the following generic type:

Fra-)Sin a: (2') where w represents the radio-frequency angularvelocity,and FU) the function, called the modulation function, which describesthe variationin amplitude of the radiofrequency wave. The variation inthe current thru the detector consequent to the application of (2) canbe calculated from (l), neglecting for simplicity, any ordinary linearimpedances which may be in the circuit. Thus The production of a currentof the type of the modulation function FU) is thus seen to depend uponthe presence of derivatives of the second and higher even orders. Theimportant thing to be noticed is that the relation etween the detectedaudio-frequency current and the impressed signal voltage is not of alower order than the second, and as an important practical result ofthis, the signal is distorted in the process of detection. Thissecond-order detection, resulting in a squarelaw response and distortionof the signal is an` inherent result of operation upon an asymmetrical,continuously curved characteristic. If the characteristic were notcontinuously curved the above power series would not be a legitimateexpansion of z' as a function of e. In order that the audio currentshall be a faithful copy of the origmodulation process must be linear;otherwise the various component vibrations in the spectrum of thecomplex sound will not be dealt with in a proportionate fashion. As wehave just shown, such a characteristic is impossible of attainment usingthe property of asymmetrical conductivity of the detector -as is thepractice of the present day.

It must be emphasized that although a silicon crystal having acontinuously curved characteristic has been taken herev as an example ofthe usual square-law detector, the same considerations and the samemathematical reasoning apply without .modification to the ionic-tubedetector, wherein the detec- 9" tion of modulated signals is broughtabout by the curvature (second and higher derivatives) of thecurrent-voltage characteristic of some conducting branchof the tube. Itis a fact familiarto most experimenters that the audio-frequencyresponse of all ionic-tube detectors is proportional to the square ofthe modulated impressed voltage over wide ranges of operation.

It can be shown mathematically that the 90 current-voltagecharacteristic of the ideal demodulator would consist of two straightlines` meeting at an angle at some definite point, which point should beused as. the operating point. With aconductor of this eccentric naturethe audio-frequency currents flowing as a result of the impressionaof amodulated radio-frequency voltage would be directly proportional to thevoltage amplitude. Experience has indicated, however, that no con- NIUductors of this ideal characterexist and no person has been able toproduce one by mcchanical or electrical combinations Continuouscurvature over a finite voltage range hasV been exhibited by theconduction characteristics 'of all such combinations which -do notfollow Ohms law. l

We have found that light metallic contacts upon a commercialferro-silicon alloy containlng about 70% to 80% silicon and 110 about20% to 30% iron, in -the crystalline form in which this alloy comes fromthe electric furnace, ield current voltage characteristics of whic 1those shown in-Fig. 2 at 1 and 2 are typical. These curves indi- '115cate that for impressed voltages in one direction (arbitrarily taken aspositive upon the diagram) the resistance is practically constant,giving a linear current-voltage characteristic; that the regions ofcurvature of the characteristics are, in general, 'very limited,extending over not more than 0.3 volt 'y about the origin; and finally,that for voltages in the reverse direction (negative) the ing straightlines. With ferro-silicon 'having more than 30% iron the inclination toeach other of the two branches of the curves gradually increases withoutincreasing the range of curvature, until at about the 50% point allcontacts lose their rectifying qualities and become simple straight-linec onductors. With ferro-silicons containing 20% iron or less the regionof curvature increases and the characteristics of sensitive contactschange gradually with lincreasing proportions of silicon into thequasi-cubic form shown for the silicon. It will be understood that theforegoing statements are based on the examination of a limited number ofsamples, and it is not excluded that ferro-silicon crystals vhaving achemical composition outside of the preferred range indicated may befound well adapted 'for the purposes of this invention. Hence the abovestatements as to the preferred constitution of the ferro-silicon are notto be regarded as restrictive of the inven- `tion. It will also belunderstood that the ferro-silicon may contain minor quantitiesof'metals or elements other than iron and silicon.

Itis rather difficult, under these conditions, when the curves dependupon the location of contact as well as upon the nature of thefundamental substance, to isolate with certainty those qualities whichdepend only upon the composition. that although the curves for differentcontacts upon a substance of a given composition may differ Widely inextent and curvature, the general shape or type of curves for allcontacts on a given substance is the same; thus y the general form ofall the curves for contacts upon commercially pure silicon (Fig. l) isthat of a cubic through the origin of coordinates, one branch of whichis displaced or distorted from the true form representing a cubicequation, thus providing a certain asymmetryabout the origin. Byinvestigating a large number of contacts upon different sampleslof theferro-silicon and upon the commercially pure silicon we have determinedconclusively that the form shown in Fig. 2 is typical of sensitive spotsupon the ferro-silicon, and that the form shown in Fig. l is typical ofsensitive spots upon the silicon, and we have never foundcharacteristics of any spots upon the ferro-silicon and the siliconwhich approached each other in form more closely than curve 3 in Fig. 1and curve l in Fig. 2.

We have also discovered that a further approach to the idealcharacteristic can be accomplished by using, in place of an iron orcopper contact point upon the ferro-silicon, v

a point of iron pyrite (FeSZ). A splinter of this substance, when heldin .light contact with a sensitive spot on the ferro-silicon,

yields volt-ampere characteristics of which the one shown at 3 in Fig. 2is typical. Ow-

It should be noted however ing t o some superposition of the surfaceconducting qualities of the pyrite and the ferrosilicon, both branchesof the characteristic curve become so nearlystraight lines that it -1sUnpossible to detect any curvature at points removed from the main bend,at the origin of coordinates. JWe have found, however, that thisdesirable feature is compensated to some extent, from a practicalstandpoint by such a decrease in the relative inclination of the twobranches of the curve that the rectifying sensitivity of the mostsensitive. point that can be located with an ironpyrite contact oint issmaller than that obtainable with t e simple iron or copper contactpoint.

Further experimental tests with radio-frequency impressed voltages haveshown that the slight departures of the characteristics of ourrectifiers from the ideal non-distorting form are insignificant for allam litudes of impressed voltage which are su stantially greater than therange of curvature of the as distinguished from the voltage-current'characteristics which show the instantaneous value of current whichflows through the rectifier as a function of the instantaneous impressedvoltage. Data for these curves were obtained by impressing upon therectifiers a measurable radio-frequency voltage, at a wave length ofapproximately 1500 meters (unmodulated) and measuring the resultingdirect component of the resulting current flowing through the rectifier.The .voltage amplitudes are plotted as abscissae, and the rectified, ordirect current as ordinates; curve l represents the results for an ironpoint contact on a ferro-silicon crystal (25% Fe), and curve 2, theresults for a contact of iron pyrite upon a similar crystal. Thesecurves indicate that the direct current response of our rectifiers tounmodulated voltage is directly proportional to the amplitude of saidvoltage throughout a range of amplitude extending from 0.3 volt to 3.5volts.` Within this range, therefore, these rectifiers behave asdistortionless demodulators to modulated alternating voltages, since themagnitude of the audio-frequency response to a modulated radiofrequencyvoltage is directly proportional to the amplitude of the envelope ofsaid voltage, and true demodulation, vor first-power rectificationresults.

In order to illustrate more clearly the existence and boundaries of theregion in which we operate our rectifiers as distortionlessdemodulators, the diagram, Fig. 4 is presented. The data for Fig. 4 werederived by computation from the experimental curves of Fig.

voltage-current characteristic. This is illus- 3. Suppose the radiosignal voltage to be demodulated is s y-'mmetrically modulated to 5012i,about its mean value. Then if' this signal voltage be so wea-k that itsmean value does not exceed 0.2 volt, its peak value does not pass out ofthe rcgionof continuous curvature on the rectifier charmteristic (Fig.and the rectifier operates as a square-law detector. the audio responsobeing proportional to the square of' the amplitude of the radio input.This condition of operation is represented bythe region AB of the Fig.-4. If the mean amplitude of the modulated input lies between 0.2 voltand 0.6 volt, the input voltage crosses from the region of curvature onto the straight portion of the rectifier characteristic in'everyradio-frequency cycle, and the law of response of the rectifier cannotbe expressed-gas a single power of the inipressed voltage; this isindicated by the transition region shown on the diagram from B to C, inwhich the simple exponential relation:

audio A"radio breaks down, and the exponent le is indeterminate. Withsignal voltages whose mean amplitude exceeds 0.6 volt, however, theaudio output is proportional to the first power of the amplitude of themodulated input; and it is in this region that We operate our rectifiersas distortionless demodiilators, represented by C to D on the diagram.Of Course the extent of this range of linear response depends, for agiven rectifier characteristic, upon the degree of modulation of theincoming signal; this region always exists, however, and we have chosenthe case of 50% modulation merely to illustrate and to define what wemean by the range of linear response with respect to the characteristicsof a particular ferro-silicon contact.

Modulated signal voltages of the order of magnitude 0.001 volt, obtainedacross the reactance elements of a receiving antenna can be detected andheard with existing practical apparatus. In order to render ourinvention useful in connection with certain weak signal voltagesobtained in practice, therefore, we combine our ferro-silicon rectifierl with means for obtaining high radio-frequency amplification of thesignal voltage before itis impressed upon the rectifier. The essentialfeature of this process is the provision that the modulated amplitude ofthis signal voltage shall be so adjusted before being impressed upon therectifier that it falls wholly within the above-defined range of linearresponse of the rectifier. For most rectifiers which fall within thescope of our invention we contemplate amplification of such an extentthat the voltage impressed upon the rectifier shall exceed 1 volt andmay exceed ten volts. We have discovered no definite upper limit to..therange of linear response with iron contacts on the ferro-silicon, andprefer therefore to employ as' high radio-frequency voltages as areconsistent with stable operation of the amplifier in order to minimizethe effects of variations in the slope of the operating characteristicof the rectifier with the location of the Contact point upon the crystalsurface. Recent improvements in cascaded ionic tube amplifiers inventedby Ballantine and described in the copending application (Serial No.629,702, filed April 3, 1923) which relate to the elimination of theeect of reaction from plate to grid circuit in every tube, make possiblethe cascading of tubes in any number of stages, so that any desiredamplification may be obtained.

The apparatus which is employed for securing the desired amplificationand distortioiiless detection of modulated radio frequency waves maytake various forms. As shown diagrammatically in Fig. 5, the demodulatorincludes a multi-stage radio frequency amplifier 10 of any appropriateconstruction from which the amplified signal wave is passed to aferro-silicon rectifier 11 having an iron or iron-pyrite contact point.The detected audio-frequency currents pass through the telephone 12which is preferably shiinted by a by-pass condenser 13. It is usuallyunnecessary to provide a bias voltage,

but when this is desired the audio-frequency circuit may include anauxiliary voltage divider 14.

In the preferred embodiment of the invention which is illustrated inFig. 6, the demodula-tor includes a radio-frequency inductive coupling15 between the multi-stage radiofrequency amplifier 10 and theferro-silicon rectifier 11.V The radio-frequency coupling preferablycomprises a vario-transformer such as describe-d in the copendingapplication of Ballantine, Serialy No. 590,514, filed September 25,1922, and its purpose is to keep currents inadvertently rectified by theamplifier out ofthe rectifier circuit. The rectifier circuit includesthe secondary of the transformer 15, the rectifier 11, and the primaryof an audio-frequency transformer 16. The primary of the transformer 16is preferably shunted by a. condenser 17, and if desired a voltagedivider 14 may be included in the rectifier circuit. The audio frequencyapparatus 18 which is connectedV across the secondary of the transformer16 may comprise any suitable arrangement of audio-frequency amplifyingunits, telephones, loud speaker, ete.

In laboratory tests with the circuit shown in Fig. 6, excellentdistortionless demodulation was obtainedwhen the radio-frequencyamplifier 10 comprises three UV201A tubes coupled Withvario-transformers and the transformer coupling 15 was also avariotransformer.

' pliication will be necessary to satisfy with any rectiier havingconstant conductivity over asubstantial range of the impressed voltage,which conductivity is different in the two directions about theoperating point, by

:adjusting the amplitude of the modulated radio-frequency wave to bringthe entire envelop of this wave within the previously defined range oflinear response. It will be apparent that little or,no radio frequencyamplification will be required when strong signals are to be received orwhen the rec-,

tiiier has such properties that its current-voltage characteristic has avery short curved portion lying between the two ranges of substantiallyuniform conductivity, i. e., the linear portions of the characteristic.When the strength of the signals is low or ,when the rectifier has suchproperties that the linear portions of the characteristic arey joined bya relatively long curve, more amthe requirement that the envelo of themodulated wave must fall within t e range of linear res onse.

lf e claim:

1. Method of operating a radio receiving system suitable for the recetion of modulated carrier-wave signals an of the type including ademodulator characterized b the fact that over a substantial range oimpressed voltages the relation between impressed voltage and outputcurrent is substantially linear, which comprises amplifying receivedsignal voltages to values within said range of linear response, andimpressing said amplied voltagesupon said demodulator.

2. Method of operating a radio receiving system suitable forthereception of carrierwave signals and of the type including a demodulatorcharacterized by the fact that over a substantial range of impressedvolta es in excess of approximately 1.0 volt the re ation betweenimpressed voltage and output current is substantially linear, whichcomprises amplifying received signal voltages to values substantially inexcess of 1.0 volt, and impressin said amplified voltages upon saiddemodu ator.

3. A radio receiving system suitable for the values within said range oflinear respo nse and means for impressing said amplified yltages of saidvalues upon said demodua or.

4. A radiol receiving system suitable for the reception of modulatedcarrier-wave signals, and comprising, in combination, a demodulatorcharacterized by the fact that over a` substantial range of impressedvolta es in excess of approximately 1.0 volt, the re ation betweenimpressed voltage and output current is substantially linear; means foramplitying received signal voltages to values substantially in excess ofl volt; and means for impressing said amplified voltages of said valuesupon said demodulator.

5. A radio receiving system suitable for the reception 'of modulatedcarrier-wave signals and comprising, in combination, ademodulatorcharacterized by the fact that'over a substantial range of` impressedpositive voltages the relation between applied voltage and output.currentl is substantially linear, over a substantial range of impressednegative voltages the relation between applied voltage and outputcurrent is substantially linear, and over a small intermediate range ofimpressed positive and negative voltages the current-voltagecharacteristic is curved, the slopes of said ilinear portions of thecurrent-voltage characteristic being different so that said linearportions are mutual- ,1y inclined; -means for amlifying received signalvoltages to values su stantially within the region represented by saidlinear branches; and means for impressing said amplified voltages ofsaid values upon said demodulator.

6. A radio receiving system suitable for the reception of modulatedcarrier-Wave signals, and comprising, in combination, a demodulatorcharacterized by the fact that over a substantial range of impressedpositive voltages in excess o approximately 1.0 volt the relationbetween applied voltage and output current is substantially linear, overa substantial range of impressed negative voltages in excess ofapproximately 1.0 volt the relation between applied voltage and outputcurrent is substantially linear, and over a small intermediate range ofimpressed positive and negative voltages lying between approximately 1.0volt positive and 1.0 volt negative the current-voltage characteristicis curved, the slopes of said linear portions of the current-voltagecharacteristic being diiferent so that said linear portions are mutuallyinclined; means Jfor amplifying received signal voltages to valuessubstantially in excess of 1 volt; and means for impressing saidvamplified voltages of said values upon said demodulator.

` 7. A radio receiving system suitable for the reception of modulatedcarrier-wave signals and comprising, in combination, a detector crystalcomposed of ferro-silicon, and characterized by the fact that itsvoltage-current characteristic comprises two mutually inclinedsubstantially linear branches joined by a relatively short curvedportion; a radio frequency amplifier .for amplifying incoming signals tovalues substantially within' the region represented by said linearbranches; and means for impressing said amplified voltages of saidvalues upon saidA detector crystal.

8. In combination, a detector comprising ferro-silicon and characterizedby the fact that its voltage-current characteristic comrises twomutually inclined substantially linear branches joined by a short curvedportion; and means for so adjusting the voltage of the incoming radiofrequency waves that it will substantlally exceed that critical valueabove which the rectified current is lineally proportional to theimpressed radio frequenc voltage.

9. detector for radio frequency oscillations, composed of an alloy ofiron and silicon between the limits of 20 percent iron- 80 percentsilicon, and v30 percent iron-70 percentJ silicon, said alloy being incrystalline form, and characterized by a voltage-current characteristiccurve having two mutually inclined substantially linear branches.

10. In a demodulator, a rectifying device comprising a crystal offerro-silicon and an iron-pyrite contact. v

In testimony whereof, we aiiix our signatures.

STUART BALLANTINE. LEWIS M. HULL.

